I bought a king size 3" thick memory foam topper, cut it in half and use it on both of our gouchos. We both love it.
You should be able to do something similar. Buy an oversize topper and cut to fit.
I think I bought it through thesportsmansguide.com
Dave
On edit: Here is the page.
Sportsman's Guide- Discount Hunting Gear, Discount Hunting Boots, Discount Shoes, Discount Ammunition, Discount Ammo, Discount Boots, Military Surplus, Outdoor Gear